Btrfs: return error code in btrfs_check_trunc_cache_free_space()

Fix to return error code instead always return 0 from function
btrfs_check_trunc_cache_free_space().
Introduced by commit 7b61cd9224
(Btrfs: don't use global block reservation for inode cache truncation)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/free-space-cache.c

@@ -213,7 +213,7 @@ int btrfs_check_trunc_cache_free_space(struct btrfs_root *root,
 	else
 		ret = 0;
 	spin_unlock(&rsv->lock);
-	return 0;
+	return ret;
 }
 
 int btrfs_truncate_free_space_cache(struct btrfs_root *root,